The present invention relates to construction equipment having a system for recovering the energy of return oil from hydraulic actuators.
The hydraulic driving type is conventionally dominant in the field of construction equipment. For instance, in one known hydraulic excavator, driving of the work implement, rotation of the upper structure, and traveling of the undercarriage are carried out by hydraulic actuators (hydraulic cylinders and hydraulic motors). Such a hydraulic excavator performs excavating operation and the like by controlling feeding of pressure oil to the hydraulic actuators from the hydraulic pump powered by the engine. Such hydraulic construction equipment has the problem of energy loss.
As an attempt to solve the problem of energy loss, the present applicant has previously proposed a hybrid hydraulic excavator in Japanese Patent Application No. 2001-068656. This hybrid hydraulic excavator is designed such that pressure oil discharged from a variable displacement hydraulic pump driven by the engine is fed to hydraulic actuators (a boom cylinder, arm cylinder, bucket cylinder etc.) through a control valve, such that there are provided a dynamo-electric motor driven by the engine and having the function of electric power generation and a battery for storing electric power generated by the dynamo-electric motor and the excessive energy of the engine is accumulated in the battery, and such that running torque is applied to the hydraulic pump by the dynamo-electric motor as required. In this hybrid hydraulic excavator, there is provided a dynamo-electric generator coupled to a hydraulic motor which is rotated by oil returning from the bottom side of the boom cylinder. When the boom is lowered, potential energy is converted to electric energy to be stored in the battery. An electric motor for rotating operation is connected to the upper structure through a decelerator. The rotating movement of the upper structure is performed by driving the rotation electric motor and during the time the rotation of the upper structure is braked, the rotation electric motor is allowed to serve as a dynamo-electric generator and the inertial energy (rotational kinetic energy) of the upper structure is converted into electric energy to be stored in the battery.
Another example of excavators capable of solving the problem of energy loss is the battery-driven excavator disclosed in Japanese Patent Publication Kokai Gazette No. 11-343642. This buttery-driven excavator is designed such that the rotating movement of the upper structure and the rising movement of the boom are carried out by direct driving by a rotation electric motor and a boom operating electric motor which are respectively activated by power from the battery, and such that the arm and the bucket which are positioned away from the gravity point of the machine are actuated by an arm cylinder and a bucket cylinder respectively, these cylinders serving as hydraulic actuators. In this battery-driven excavator, when laying the boom down, the boom operating electric motor is activated as a dynamo-electric generator and the potential energy of the boom is converted into electric energy to be stored in the battery. When braking the rotation of the upper structure, the rotation motor is allowed to function as a dynamo-electric generator and the inertial energy (rotational kinetic energy) of the upper structure is converted into electric energy to be stored in the buttery. Thus, continuous operating time can be effectively increased for energy saving, while using the battery as a driving source.
The hybrid hydraulic excavator and battery-driven excavator, which have been described above, present such a drawback that the energy of return oil produced in the arm cylinder and the bucket cylinder cannot be recovered. In the case of the hybrid hydraulic excavator, although it is conceivable to enable energy recovery by letting the return oils from the arm cylinder and the bucket cylinder flow in the hydraulic motor in which the return oil from the boom cylinder is flowing, there is the possibility of a failure in efficient recovery of the energy of the return oils due to the influence of back flows which may occur owing to the difference between the pressures of the return oils from the respective cylinders.
In addition, since the output torque per unit weight of electric motors is generally small compared to hydraulic actuators, it is necessary to increase the capacity of the rotation electric motor in the case of the hybrid hydraulic excavator and to increase the capacities of the rotation electric motor and the boom operating electric motor in the case of the battery-driven excavator. This causes a problem in terms of configuration and cost performance. Additionally, in the case of power failure, it is difficult to assure safety.
The present invention is directed to overcoming the foregoing problems and a primary object of the invention is therefore to provide compact, inexpensive construction equipment capable of effectively recovering energy from a plurality of hydraulic actuators.
In a first aspect of the invention, the above object can be accomplished by construction equipment having a plurality of hydraulic actuators, the construction equipment further comprising:
a hydraulic recovery circuit for recovering return oil from the hydraulic actuators, the circuit being equipped with a rotating machine rotatably driven by the recovered return oil;
oil guide passages each of which is for guiding return oil from the meter-out side of its corresponding hydraulic actuator to the hydraulic recovery circuit;
switching means for switching the respective oil guide passages between their opened and closed states;
pressure detecting means for detecting the back pressure of each hydraulic pressure actuator; and
a dynamo-electric generator for generating electric power from the rotary force of the rotating machine.
According to the invention, when recovering return oil from the meter-out side of one of the hydraulic actuators, the corresponding oil guide passage is opened by the switching operation of the switching means so that the return oil is guided to the hydraulic recovery circuit through the guide oil passage. Then, the rotating machine is rotatably driven by the return oil which has been guided to the hydraulic recovery circuit, so that the dynamo-electric generator generates electric power. Thus, the hydraulic energy of the return oil from the hydraulic actuator can be recovered as electric energy. With this arrangement, driving of a work implement etc. and energy recovery can be carried out by a compact and low-cost system without use of a large-capacity electric motor such as used in the prior art. When recovering return oils from the plurality of hydraulic actuators at the same time, the back pressure of a hydraulic actuator which has been found to be the highest by the pressure detecting means is preferentially selected, and the switching means performs its switching operation so as to guide the return oil associated with the selected back pressure to the hydraulic recovery circuit. In this way, the return oil from a hydraulic actuator having the highest energy recovery efficiency is selectively guided to the hydraulic recovery circuit and the hydraulic energy of this return oil is recovered as electric energy. As a result, a drop in the energy recovery rate due to the interference of the return oils from the plurality of hydraulic actuators can be prevented and effective energy recovery can be assured.
In a second aspect of the invention, there is provided construction equipment having a plurality of hydraulic actuators, wherein at least two sets of hydraulic energy recovering means are provided, each hydraulic energy recovering means comprising:
a hydraulic recovery circuit for recovering return oil from the hydraulic actuators, the circuit being equipped with a rotating machine rotatably driven by the recovered return oil;
oil guide passages each of which is for guiding return oil from the meter-out side of its corresponding hydraulic actuator to the hydraulic recovery circuit;
switching means for switching the respective oil guide passages between their opened and closed states;
pressure detecting means for detecting the back pressure of each hydraulic pressure actuator; and
a dynamo-electric generator for generating electric power from the rotary force of the rotating machine.
The construction equipment according to the second aspect of the invention has, of course, the same function and effect as the construction equipment having the first aspect of the invention. Moreover, since it has at least two sets of hydraulic energy recovering means and each means is arranged such that the return oil of a hydraulic actuator having the highest energy recovery rate is selectively guided to the hydraulic recovery circuit and the hydraulic energy of the return oil is recovered as electric energy, it can recover a larger amount of energy than the construction equipment having the first aspect of the invention does.
Preferably, in both of the first and second aspects, the switching means has a controller and the controller is designed as follows. During single actuator operation in which one of the plurality of hydraulic actuators is independently activated, the controller controls the switching operation of the switching means such that the return oil from the activated hydraulic actuator is guided to the hydraulic recovery circuit. During complex actuator operation in which the plurality of hydraulic actuators are activated at the same time, the controller controls the switching operation of the switching means such that the back pressures of the hydraulic actuators detected by the pressure detecting means are compared to select the highest one and the return oil associated with the selected back pressure is guided to the hydraulic recovery circuit. This makes it possible to attain the desired effect with a comparatively simple hydraulic circuit configuration.